Measure the voltage between the positive side of the alternator and the positive terminal at the battery...you should see well under 1V IIRC. If you see more than that, then the voltage isn't getting from the alternator to the battery and you need to figure out why. Could be a bad positive lead or a corroded connector someplace, or the alternator itself is bad. Just because you see 14V at the alternator with the car at idle doesn't mean the alternator is good...it could be failing under load and thus the battery goes dead.